Can this be solved in polynomial time?

I have M bit-strings with N bits each. I need to find K strings, so
that logical OR of all the chosen strings has the most bits set.

Or in other words:
You have a set S of size N.
You are given M subsets of S.
Find K of the given subsets, so that their union is of maximal size.

Can this problem be solved efficiently? If so, how?

Thanks

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